FIG. 1 shows an exemplary network structure of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) as a mobile communication system to which a related art and the present invention are applied. The E-UTRAN system is a system that has evolved from the UTRAN system, and its standardization work is currently being performed by the 3GPP standards organization. The E-UTRAN system can also be referred to as a LTE (Long-Term Evolution) system.
The E-UTRAN generally comprises base stations (i.e., eNode B or eNB), and each eNB is connected by X2 interfaces. The eNBs are connected to terminals (User Equipment; UE) through a radio interface, and are connected to an Evolved Packet Core (EPC) through S1 interfaces.
The various layers of the radio interface protocol between the mobile terminal and the network may be divided into a layer 1 (L1), a layer 2 (L2) and a layer 3 (L3), based upon the lower three layers of the Open System Interconnection (OSI) standard model that is well-known in the field of communications systems. Among these layers, Layer 1 (L1), namely, the physical layer, provides an information transfer service to an upper layer by using a physical channel, while a Radio Resource Control (RRC) layer located in the lowermost portion of the Layer 3 (L3) performs the function of controlling radio resources between the mobile terminal and the network. To do so, the RRC layer exchanges RRC messages between the mobile terminal and the network.
FIGS. 2 and 3 show radio interface protocol architecture between a terminal and E-UTRAN based on 3GPP radio access network standards. Particularly, FIG. 2 shows radio protocol architecture in a control plane, and FIG. 3 shows radio protocol architecture in a user plane.
The radio interface protocol in FIGS. 2 and 3 has horizontal layers comprising a physical layer, a data link layer and a network layer, and has vertical planes comprising a user plane for transmitting user traffic and a control plane for transmitting control signals. The protocol layers in FIGS. 2 and 3 can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) standard model widely known in communications systems. Hereinafter, each layer in the radio protocol control plane in FIG. 2 and a radio protocol user plane in FIG. 3 will be described.
A first layer, as a physical layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to its upper layer, called a Medium Access Control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. Data is transferred via a physical channel between different physical layers, namely, between the physical layer of a transmitting side and the physical layer of a receiving side. The physical channel is modulated based on an Orthogonal Frequency Division Multiplexing (OFDM) technique, and utilizes time and frequency as radio resources.
The MAC layer located at the second layer provides a service to an upper layer, called a Radio Link Control (RLC) layer, via a logical channel. The RLC layer of the second layer supports reliable data transmissions. The function of the RLC layer may be implemented as a functional block in the MAC layer. In this case, the RLC layer may not exist. A Packet Data Convergence Protocol (PDCP) layer of the second layer, in the radio protocol user plane, is used to efficiently transmit IP packets, such as IPv4 or IPv6, on a radio interface with a relatively narrow bandwidth. For this purpose, the PDCP layer reduces the size of an IP packet header which is relatively great in size and includes unnecessary control information, namely, a function called header compression is performed.
A Radio Resource Control (RRC) layer located at the lowest portion of the third layer is only defined in the control plane. The RRC layer controls logical channels, transport channels and physical channels in relation to establishment, re-configuration and release of Radio Bearers (RBs). Here, the RB signifies a service provided by the second layer for data transmissions between the terminal and the E-UTRAN. If an RRC connection is established between the RRC layer of the terminal and the RRC layer of the radio network, the terminal is in the RRC connected mode. Otherwise, the terminal is in an RRC idle mode.
A Non-Access Stratum (NAS) layer located at an upper portion of the RRC layer performs functions, such as session management, mobility management and the like.
One cell constructing an eNB is set to one of bandwidths of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, 20 MHz and the like, so as to provide downlink or uplink transmission services to multiple terminals. Here, different cells may be set to provide different bandwidths.
Downlink transport channels for transmitting data from a network to a terminal may comprise a Broadcast Channel (BCH) for transmitting system information, a Paging Channel (PCH) for transmitting paging messages and a downlink Shared Channel (SCH) for transmitting other user traffic or control messages. Traffic or control messages of a downlink point-to-multipoint service (multicast or broadcast service) may be transmitted either via a downlink SCH, or via a separate downlink Multicast Channel (MCH). In addition, uplink transport channels for transmitting data from a terminal to a network may comprise a Random Access Channel (RACH) for transmitting an initial control message and an uplink Shared Channel (SCH) for transmitting user traffic or control messages.
Logical channels which are located at an upper portion of transport channels and mapped to the transport channels include a Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH), a Common Control Channel (CCCH), a MBMS point-to-multipoint Control Channel/Multicast Control Channel (MCCH), a MBMS point-to-multipoint Traffic Channel/Multicast Traffic Channel (MTCH), and the like.
FIG. 4 shows a related art control channel transmission.
The physical channel is comprised of a plurality of sub-frames on a time axis and a plurality of sub-carriers on a frequency axis. Here, one sub-frame is comprised of a plurality of symbols on the time axis. One sub-frame is comprised of a plurality of resource blocks (RBs), and a resource block is comprised of a plurality of symbols and a plurality of sub-carriers. In addition, each sub-frame may use specific sub-carriers of specific symbols (e.g., a first symbol) of a corresponding sub-frame for the Physical Downlink Control Channel (PDCCH), that is, the L1/L2 control channel. One sub-frame has a time duration of 0.5 ms, and a Transmission Time Interval (TTI) indicating a unit of time that data is transmitted has a time duration of 1 ms, corresponding to 2 sub-frames.
The wireless network may perform a counting process to check the presence of terminals that receive a specific point-to-multipoint service in a specific cell or to count the number of terminals. The point-to-multipoint service counting process refers to a process, in which if the wireless network transmits an access information message to the terminal via an MCCH channel, the terminal transmits an RRC Connection request message or a Cell Update message in response to the access information message.
In the related art, the MCCH channel (i.e., control channel) periodically transmits control information in every modification period, and the terminal determines that the control information transmitted from the MCCH channel has been modified in every modification period. Accordingly, the RLC layer of the terminal which receives the MCCH channel would discard RLC SDUs which were not completely received in a previous modification period. Even though the control information is always not modified in every modification period, the RLC layer of the terminal discards the RLC SDUs that were completely received in every modification period, thus to cause a great problem of inefficiently receiving such unmodified control information.